1. Field of the Invention
This invention relates to a liquid crystal display device and its manufacturing method especially suitable for application to a liquid crystal display having an electrically-conductive light-shading layer provided in a level above a thin film transistor for driving pixel electrodes and below the pixel electrodes.
2. Description of the Related Art
Liquid crystal display devices are widely used as flat-type displays. As a thin film transistor (TFT) for driving pixel electrodes in such a liquid crystal display, amorphous silicon (a-Si) TFT was used conventionally. Recently, however, polycrystalline SiTFT has come to be used often.
Photosensitivity of polycrystalline SiTFT is not so high as that of a-SiTFT. Recently, however, liquid crystal display devices such as projector have increasingly been used under intensive light, and light leak current is no more negligible even with polycrystalline SiTFT. As a result, degradation of contrast and deterioration of the image quality such as cross-talk and flicker, for example, have arisen as problems.
In liquid crystal display devices, light from light source is usually introduced from the side of the opposed substrate. As to prevention of light from entering into polycrystalline SiTFT, as disclosed in Japanese Patent Laid-Open Publication No. hei 5-100250 and Japanese Patent Application No. hei 10-307465, for example, by locating the electrically-conductive light-shading layer (black matrix) conventionally provided on the opposed substrate to the level above the polycrystalline SiTFT of a TFT substrate, which is nearer to the polycrystalline SiTFT, reduction of such light has been attained.
However, according to the knowledge of the inventor, in the techniques disclosed in the documents, Japanese Patent Laid-Open Publication No. hei 5-100250 and Japanese Patent Application No. hei 10-307465, due to the phenomenon that the thickness of the electrically-conductive light-shading layer becomes thinner in level-difference portions caused by unevenness of the underlying insulating layer, in other words, the phenomenon that the step coverage degrades, the shading performance is insufficient at the level-difference portions. Therefore, under high-luminance irradiation of light, leak light from level-difference portions causes generation of a light leak current, and deterioration of the image quality cannot be prevent under the current technologies.
This problem is discussed below in greater detail. FIG. 1 shows a TFT substrate of a conventional active matrix type liquid crystal display device. As shown in FIG. 1, a shading layer 102 is provided on a shading region of a quartz glass substrate 101, and an inter-layer insulating film 103 is provided to cover the shading layer 102. Formed on the inter-layer insulating film 103 is a polycrystalline Si film 104 of a predetermined pattern, and a gate insulating film 105 is provided to cover the polycrystalline Si film 104. A gate wiring 106 is formed on the gate insulating film 105. Although not shown, the polycrystalline Si film 104 has formed therein a source region and a drain region (not shown) in self alignment with the gate wiring 106. The gate electrode made of the gate wiring 106 and those source region and drain region make up a polycrystalline SiTFT for driving pixel electrodes. On a predetermined portion of the gate insulating film 105 above the drain region, an electrode 107 is provided. This structure interposing the gate insulating film 105 between this electrode 107 and the drain region constitutes a holding capacitor element.
An inter-layer insulating film 108 is provided to cover the gate wiring 106 and the electrode 107. Contact holes 109 and 110 are formed at predetermined portions of the inter-layer insulating film 108 and the gate insulating film 105. In the shading region, a lead-out electrode 111 is provided in connection with the drain region of the polycrystalline SiTFT through the contact hole 109, and a signal wiring 112 is provided in connection with the source region of the polycrystalline SITFT through the contact hole 110. An inter-layer insulating film 113 is formed to cover these lead-out electrode 111 and the signal wiring 112. In a predetermined location on the inter-layer insulating film 113, a SiN film 114 made by plasma CVD lies. The SiN film 114 mainly inactivates dangling bonds in the polycrystalline Si film 104 with hydrogen, and functions as a hydrogen supply source for improving the property of the polycrystalline SiTFT. Further provided is a contact hole 115 in a predetermined portion of the inter-layer insulating film 113 above the lead-out electrode 111. In contact with the lead-out electrode 111 through the contact hole 114, an electrically-conductive light-shading layer 116 is provided on the inter-layer insulating film 113, and an electrically-conductive light-shading layer 117 is provided on the SiN film 114. The structure stacking these electrically-conductive light-shading layer 116, 117, lead-out electrode 111 and signal wiring 112 shields the incident light from above over the entire region other than the pixel aperture region. An inter-layer insulating film 118 is provided to cover the electrically-conductive light-shading layers 116, 117. The inter-layer insulating film 118 has formed a contact hole 119 in a predetermined location above the electrically-conductive light-shading layer 116. On the inter-layer insulating film 118, a transparent pixel electrode 120 is provided in connection with the electrically-conductive light shading layer 116 through the contact hole 119. An orientation film 121 of a liquid crystal (not shown) is provided to cover the pixel electrode 120.
In the conventional liquid crystal display apparatus explained above with reference to FIG. 1, since the electrically-conductive light-shading layers 116, 117 are formed on the inter-layer insulating film 113 which includes a large unevenness reflecting the stepped configuration of the base layer, step coverage of these electrically-conductive light-shading layers 116, 117 degrades. Therefore, the light shading performance of these electrically-conductive light-shading layers 116, 117 was not sufficient in these step portions, which invited generation of a light leak current by leak light from level-difference portions under high-luminance irradiation of light, and deterioration of the image quality could not be prevented.
It is therefore an object of the invention to provide a liquid crystal display device and its manufacturing method which can improve the light shading performance of an electrically-conductive light shading layer and can prevent deterioration of the image quality caused by a light leak current.
According to the invention, there is provided a liquid crystal display device having a thin-film transistor for driving a pixel electrode on a substrate and an electrically-conductive light-shading layer lying in a level above the thin film transistor and below the pixel electrode, comprising:
the electrically-conductive light-shading layer being formed on a smoothed layer.
There is further provided a liquid crystal display device having a first light-shading layer formed on a substrate, a thin film transistor for driving a pixel electrode formed on the first light shading layer, and a second light-shading layer formed in a level above the thin-film transistor and below the pixel electrode, comprising:
the second light-shading layer being formed on a smoothed layer.
There is further provided a method for manufacturing a liquid crystal display device having a thin-film transistor for driving a pixel electrode on a substrate and an electrically-conductive light-shading layer lying in a level above the thin film transistor and below the pixel electrode, characterized in:
the electrically-conductive light-shading layer being formed on a smoothed layer.
There is further provided a method for manufacturing a liquid crystal display device having a first light-shading layer formed on a substrate, a thin film transistor for driving a pixel electrode formed on the first light shading layer, and a second light-shading layer formed in a level above the thin-film transistor and below the pixel electrode, characterized in:
the second light-shading layer being formed on a smoothed layer.
In the present invention, the surface of the smoothed layer is smoothed to a residual level difference (difference between the highest level and the lowest level) not larger than 0.5 xcexcm and more preferably not larger than 0.3 xcexcm, excluding the contact portions in the display region. The smoothed layer is typically an insulating layer made of SiO2 as its major component, but it may be an insulating layer of any other appropriate material.
To make the smoothed layer, various methods are usable, which can ensure a residual level difference not larger than 0.5 xcexcm, for example. Examples are a method for making a film ensuring a good burying property, such as plasma CVD or normal-pressure CVD, using tetraethoxysilane (TEOS) as the source material gas, a method first making a film of phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like and then making it reflow, a flow method utilizing spin-on-glass (SOG), a method first making an insulating film and thereafter conducting its etch-back, a method first making an insulating film and then polishing it by chemical-mechanical polishing (CMP), and so on. Among these methods, CMP is preferable because of its advantages that excellent evenness is ensured and plasma damage to thin film transistors can be prevented. Usable as the insulating layer to be smoothed by CMP are, especially, a film by plasma CVD using TEOS, film by atmospheric-pressure CVD using TEOS, film by high-density plasma CVD, multi-layered film of these layers, and so on.
From the viewpoint of restricting the coupling capacitance with the adjacent wiring, the electrically-conductive light-shading layer preferably has a sheet resistance not higher than 100 xcexa9/xe2x96xa1 and more preferably not higher than 10 xcexa9/xe2x96xa1. Furthermore, from the viewpoint of suppressing the light leak current of the thin film transistor, the electrically-conductive light-shading layer preferably exhibits a transmittance not higher than 10% for light of a wavelength from 400 to 500 nm, and more preferably not higher than 5%, and yet lower for increasing the light shading effect. Basically, thickness of the electrically-conductive light shading layer may be selected freely as long as it ensures both requirements for lower sheet resistance and light shading property. Actually, however, since a transparent pixel electrode is further formed on the electrically-conductive light shading layer via an insulating layer to sandwich a liquid crystal, the thickness of the electrically-conductive light shading layer is preferably selected within a range ensuring that any unevenness caused by the electrically-conductive light shading layer does not adversely affects the orientation of the liquid crystal. Practically, thickness of the electrically-conductive light-shading layer is preferably within the range from 50 to 500 nm, and more preferably from 100 to 300 nm. Basically, any material is freely selected as the material of the electrically-conductive light-shading layer as far as both it satisfies both an electrical conductivity and a light shading property. Appropriate examples are, for example, Al, Cu, W, Mo, Pt, Pd, Ti, TiN, Cr, their alloys or silicides, and so on.
The electrically-conductive light-shading layer is provided in the pixel portion, for example, as two separate portions, one being connected to the pixel electrode and the other connected to the common potential. Against incident light coming from above, the electrically-conductive light-shading layer makes multiple layers with at least one other light-shading Layer to shade the light over the entire area other than the regions of pixel openings.
In the present invention, the thin film transistor for driving the pixel electrode is typically a thin film transistor made of polycrystalline silicon, namely, polycrystalline SiTFT. This polycrystalline SiTFT may be either high-temperature polycrystalline SiTFT using a polycrystalline Si film made by a high-temperature process or low-temperature polycrystalline SiTFT using a polycrystalline Si film made by a low-temperature process. Alternatively, the thin film transistor for driving the pixel electrode may be a-SiTFT.
According to the invention having the above-summarized structure, since the electrically-conductive light-shading layer or the second light shading layer is formed on a smoothed layer, step coverage of the electrically-conductive light-shading layer or the second light shading layer is improved, and the evenness of its thickness is improved. Therefore, the electrically-conductive light-shading layer or the second light shading layer performs a sufficient light shading function, leak light can be reduced remarkably. As a result, even under high-luminance irradiation of light, generation of a light leak current can be prevented.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.